If you click 'Restore Purchase', and nothing happens, we have identified a bug primarily affecting 10.13 High Sierra users with a "clean slate" operating system. For example, setting up a brand new computer or after a recent upgrade. We have filed a ticket with Apple as a bug, and this is affecting many other products as well (for example, see Hype 3).
First, it's important to know this only affects users that downloaded the "free" version and then bought the license via in-app purchase. There would have been an accompanying AppStore receipt with "MSG Viewer for Outlook, License" on it. (*If you purchased another version of our product, like MSG Viewer for Outlook Pro or directly from us via our website, do not follow this procedure).
The resolution is to purchase the software again (do not worry, you will not be billed twice). It will appear to charge you for the software, and then after you accept the purchase it will tell you it's already been purchased and will be restored for free. Apple blocks duplicate in-app purchases, so this is their built-in workflow. Provided that you used the same iCloud account as the one when you originally bought the software, this will work.
As always, please feel free to reach out to our technical support team with any questions or difficulties.